Apparatus and method for forward error correction over a communication channel

ABSTRACT

There are various drawbacks by using existing OTN (Optical Transport Network) frames for communication between OTN cards. Such drawbacks might for example include high latency, low robustness, and/or high coding rate. According to embodiments of the present disclosure, systems and methods are provided for modifying an OTN frame (or creating a new frame with data from the OTN frame) prior to transmission by an OTL (Optical channel Transport Lane) in order to address some or all of the foregoing drawbacks. Note that this embodiment can make use of existing hardware (e.g. hardware used for generating the OTN frame, and the OTL used for transmission).

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/211,938 filed on Mar. 14, 2014, the entire contents of which areincorporated by reference. This application claims priority from U.S.Provisional Patent Application No. 61/791,697 filed on Mar. 15, 2013,the entire contents of which are incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to communication systems, and moreparticularly to communicating data over a communication channel.

BACKGROUND

A transmitter can transmit a signal to a receiver over a communicationchannel such as a backplane. The signal can encode various data. If thecommunication channel is not perfect, then the signal received by thereceiver may not be identical to the signal transited by thetransmitter. For instance, the communication channel may have introducednoise into the signal. If the signal received by the receiver isdistorted enough by the communication channel, then the data may not berecoverable by the receiver.

FEC (Forward Error Correction) or channel coding is a technique thatinvolves encoding a signal in a redundant manner so that data sent withthe signal may be recoverable even if there are errors in transmission.Such techniques may be employed when it is difficult or impractical tosupport re-transmission of data following an error. RS (Reed Solomon)encoding is an example FEC. Other FECs exist.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described withreference to the attached drawings in which:

FIG. 1 is a block diagram of OTN (Optical Transport Network) systemhaving first and second OTN cards connected via an OTN backplane;

FIGS. 2A through 2D are example OTN frames that may be communicatedbetween the OTN cards shown in FIG. 1;

FIG. 3 is a block diagram of an FEC encoder/decoder fortransmitting/receiving data over an OTL (Optical channel TransportLane), in accordance with various embodiments of the present disclosure;

FIG. 4 is a schematic showing details of frame generation in accordancewith a first embodiment of the present disclosure for a case of nocodeword interleaving by the OTL;

FIG. 5 is a schematic showing details of frame generation in accordancewith the first embodiment of the present disclosure for a case ofcodeword interleaving by the OTL; and

FIG. 6 is a schematic showing details of data processing by the OTL forthe first embodiment of the present disclosure;

FIG. 7 is a schematic showing details of bit shuffling by the OTL withcodeword interleaving for the various embodiments of the presentdisclosure;

FIG. 8 is a schematic showing details of symbol striping for a case ofcodeword interleaving by the OTL, in accordance with the firstembodiment of the present disclosure;

FIG. 9 is a schematic showing details of symbol striping for a case ofno codeword interleaving by the OTL, in accordance with the firstembodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

It should be understood at the outset that although illustrativeimplementations of one or more embodiments of the present disclosure areprovided below, the disclosed systems and/or methods may be implementedusing any number of techniques, whether currently known or in existence.The present disclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated below, includingthe exemplary designs and implementations illustrated and describedherein, but may be modified within the scope of the appended claimsalong with their full scope of equivalents.

There are various drawbacks by using existing OTN frames forcommunication between OTN cards. Such drawbacks might for exampleinclude high latency, low robustness, and/or high transmission rate.According to embodiments of the present disclosure, systems and methodsare provided for modifying an OTN frame (or creating a new frame withdata from the OTN frame) prior to transmission by an OTL in order toaddress some or all of the foregoing drawbacks. Note that thisembodiment can make use of existing hardware (e.g. hardware used forgenerating the OTN frame, and the OTL used for transmission). Otheraspects and features of the present disclosure will become apparent, tothose ordinarily skilled in the art, upon review of the followingdescription of the specific embodiments of the invention.

Introduction

Referring first to FIG. 1, shown is a block diagram of an OTN system 100having first and second OTN cards 110,120 connected via an OTN backplane130. Each OTN card 110,120 has an OTN framer device 111,121 or a switchdevice 121 as shown in the illustrated example. As an alternative to theswitch device 121, there may be an FPGA (Field Programmable Gate Array)and/or other circuitry not specially shown. The block diagram of FIG. 1is a simplified and generalized view of an OTN DWDM (Dense WavelengthDivision Multiplexing) system and is applicable to many differentapplications: transponder or muxponder applications (e.g. both of theOTN cards 110,120 are OTN line cards interconnected through the OTNbackplane 130), optical transmission (e.g. the first OTN card 110 on theleft contains the OTN framer device 111 and the OTN card 121 on theright contains circuitry for optics), or even an OTN cross-connect (e.g.the first OTN card 110 on the left is an OTN line card and the secondOTN card 120 on the right is an OTN switch card having the switch device121).

Each OTN framer device 111,121 (or switch device, FPGA, other circuitry,etc.) is configured to generate OTN frames, which can be transmittedacross the backplane 130 thereby enabling communication between the OTNcards 110,120. One OTN framer device 111 can operate as a transmitter(i.e. encoding an FEC) with the other OTN framer device 121 operating asa receiver (i.e. decoding the FEC). Note that both OTN framer devices111,121 may be capable of operating as a transmitter and a receiver. Insome implementations, both OTN framer devices 111,121 contain high-speedinterfaces (e.g. analog design) to serialize information at high speedrates (e.g. ˜27 Gbps). In some implementations, the OTN framer deviceshave a DFE (Decision Feedback Equalizer) that can be used to implementthe high-speed interface for receiving data.

Referring now to FIGS. 2A through 2D, shown are example OTN frames thatmay be communicated between the OTN cards. OTUk (Optical channelTransport Unit¹) frames, as standardized by ITU (InternationalTelecommunication Union) G.709² and shown in FIGS. 2A and 2B, use RS(255,238, m=8, t=8) FEC providing a 6.7% FEC overhead redundancy.Further details are provided in Annex A of ITU G.709. Appendix II of ITUG.709 provides multiple generic examples of functionally standardizedOTU frame structures (OTUkV) for illustrative purpose with different FECredundancy. For example, in Appendix II of ITU G.709, there is a case ofan OTUk with a larger OTUkV FEC, as per FIG. 2C, where X is assumed tobe greater than 4080. There is also a case of an OTUk with a smallerOTUkV FEC and remainder of FEC area filled with fixed items as shown inFIG. 2D. ¹ k (=1, 2, 3, 4) indicates rate (e.g. OTU1=10 Gbps, OTU2=10Gbps, OTU3=40 Gbps, OTU4=100 Gbps)² ITU-T Recommendation G.709“Interfaces for the Optical Transport Network (OTN)” (02/2012) isincorporated by reference in its entirety.

However, there are various drawbacks by using the existing OTN framesfor communication between the OTN cards. As shown in the illustratedexamples, each OTN frame has FEC parity at the end of each row of theOTN frame. This can result in poor latency because all data in a rowwould be received before any processing with the FEC parity can bestarted. For instance, for some backplane applications (e.g. transportof Fibre Channel client signals), GFEC (G.709 FEC) latency is too large.Another drawback is the interaction between the G.709 FEC and theOTLk.n, which can result in poor robustness if there are burst errors.For instance, when a standard G.709 FEC-RS (255, 239, m=8, t=8)—iscombined with OTL, the result is very low tolerance to correlated errors(i.e. a small burst can corrupt many symbols of the same codeword, thussignificantly lowering Net Coding Gain). Another drawback of using theG.709 FEC over backplane is that its coding rate is relatively high(6.7%), reducing the maximum backplane length.

One approach could be to modify the OTN framer devices and the OTL in anattempt to address some or all of the foregoing drawbacks. However,since the OTN framer devices and the OTL already exist, this approachmay not be desirable. Many 100G OTN applications (OTU4) and future“beyond 100G” OTN applications (OTUCn) may one day use an FEC method forbackplanes, but an FEC method designed for an OTN backplane has not yetbeen developed and standardized.

According to embodiments of the present disclosure, systems and methodsare provided for modifying an existing OTN frame (or creating a newframe with data from the OTN frame) prior to transmission by the OTL inorder to address some or all of the foregoing drawbacks with theexisting OTN frames. Note that these embodiments can make use ofexisting hardware (e.g. reuse the OTN framer devices and/or the OTL).

FEC Encoder/Decoder: Overview

Referring now to FIG. 3, shown is a block diagram of an FECencoder/decoder 300 in accordance with an embodiment of the presentdisclosure. The FEC encoder/decoder 300 has a transmit path310,311,312,313,314 corresponding to a transmitting OTL 315, and areceive path 320,321,322,323,324 corresponding to a receiving OTL 325.The transmit path 310,311,312,313,314 includes:

-   -   circuitry 310 for removing FEC parity from an OTN frame (and        optionally removing OTN FAS bytes—Frame Alignment Signal),    -   circuitry 311 for re-structuring data from the OTN frame and        creating FEC parity locations (and optionally inserting the        backplane FAS bytes if they were removed by the circuitry 310)        for a new backplane frame format,    -   circuitry 312 for bit shuffling the data,    -   circuitry 313 for coding the data to determine the FEC parity,        and    -   circuitry 314 for bit de-shuffling the data prior to the new        frame being transmitted by the OTL.        The receive path 320,321,322,323,324 has circuitry that is        complementary to the transmit path 310,311,312,313,314.

Data is coded in such a way that compensates for bit shuffling done bythe OTL 315. To this end, the circuitry 312 bit shuffles the data priorto the coding, and the circuitry 314 bit de-shuffles the data after thecoding. In some implementations, this is performed such that there isstriping at a symbol level across the backplane lanes as will bedescribed in further detail below. In the illustrated example, thecircuitry 313 for coding the data is an RS encoder 313, althoughalternative encoders other than the RS encoder 313 may be possible.

There are many possible codes that can be used by the RS encoder 313. Ina “first embodiment”, the RS encoder uses an RS (627,613, m=10, t=7)code. In a “second embodiment”, the RS encoder uses an RS (528,514,m=10, t=7) code. In a “third embodiment”, the RS encoder uses an RS(970,956, m=10, t=7) code. These three embodiments will be described infurther detail below. Other embodiments are possible and are within thescope of the present disclosure.

In some implementations, the OTN FAS bytes are not removed and arereused directly as FAS bytes for the backplane frame. However, in otherimplementations, the OTN FAS bytes are removed by the circuitry 310 andinserted by the circuitry 311. Other implementations are possible andare within the scope of the present disclosure.

Further details of the transmit path 310,311,312,313,314 and itsoperation, including further details of the “first embodiment”, “secondembodiment” and “third embodiment” noted above, are provided withreference to the subsequent Figures. The receive path320,321,322,323,324 and its operation are complementary to the transmitpath 310,311,312,313,314 and its operation. Thus, further details of thereceive path 320,321,322,323,324 and its operation may not be explicitlyprovided. However, it is to be understood that the receive path320,321,322,323,324 and its operation are within the scope of thepresent disclosure.

FEC Encoder/Decoder: First Embodiment

According to the first embodiment, data of an OTN frame (for example, anOTN frame with a 255/239 FEC redundancy) is re-structured into anotherOTN frame (the backplane frame) with a 245/239 FEC redundancy. Thebackplane frame is structured similarly to a standard OTN frame: 245column groups and four rows. Twenty RS (627,613, m=10, t=7) codewordsare mapped into each backplane frame (five codewords per row). A FIFO isused for the re-structuring. On each backplane frame row, the fivecodewords can be mapped using different methods, examples of which aredescribed below. Note that other methods are possible and are within thescope of the present disclosure.

A first method involves mapping the five codewords on a sequential(non-interleaved) fashion. A schematic showing details of framegeneration in accordance with the first embodiment is shown in FIG. 4.The frame generation involves circuitry 410 for removing FEC parity froman OTN frame 401 (and optionally removing OTN FAS bytes—Frame AlignmentSignal), and circuitry 411 for re-structuring data from the OTN frame401 and creating FEC parity locations (and optionally inserting thebackplane FAS bytes if they were removed by the circuitry 410) for a newbackplane frame format 402.

A second method involves mapping the five codewords on asymbol-interleaved fashion. A schematic showing details of framegeneration in accordance with the first embodiment is shown in FIG. 5.The frame generation involves circuitry 510 for removing FEC parity froman OTN frame 501 (and optionally removing OTN FAS bytes—Frame AlignmentSignal), and circuitry 511 for re-structuring data from the OTN frame501 and creating FEC parity locations (and optionally inserting thebackplane FAS bytes if they were removed by the circuitry 510) for a newbackplane frame format 502.

Note that the re-structuring of data can reduce latency (even though theFIFO might introduce some latency) because there is less data to receivebefore processing with FEC parity can be started. Note that the FECparity from the OTN frame is discarded, as new FEC parity for the 20codewords is calculated with RS because of the restructuring of thedata.

FIGS. 4 and 5 show conversion of a standard OTN frame 401,501 having 255column groups per OTN frame row into a proprietary OTN frame 402,502having 245 column groups per OTN frame row. The number of availableparity bits per OTN from row for the standard OTN frame is 16×128 bits.The number of available parity bits per OTN from row for the proprietaryOTN frame 402,502 is 6×128 bits (i.e. 245−239=6).

According to the first embodiment, five RS (627,613, t=7, m=10)codewords are mapped into each row of the OTN backplane frame. Thisresults in the following length mismatch:

-   -   Available parity=6×128 bit=768 bits    -   Parity to use=5×140 bit=700 bits    -   Protected data=5×613×10 bits=30,650 bits    -   Data to protect=239×128 bits=30,592 bits        To address the length mismatch in parity, the 68 (i.e. 768−700)        unused parity bits are split into two groups:    -   58 bits (i.e. 30,650−30,592) are used as a protected user        channel, and    -   the remaining 10 bits (i.e. 68−58) are used as an unprotected        user channel.

There are special considerations for the FEC parity calculation with RSbecause the OTL involves bit shuffling. With reference back to the FECencoder/decoder 300 shown in FIG. 3, there is circuitry 312 for bitshuffling before RS encoding and circuitry 314 for bit re-shufflingafter the RS coding. The bit shuffling and the bit re-shuffling isperformed in a manner that compensates for the bit shuffling done by theOTL. In some implementations, this is performed such that there issymbol striping across multiple lanes despite the bit shuffling done bythe OTL. The bit shuffling by the OTL will be explained in furtherdetail with reference to FIGS. 6 and 7 for an example case of 5×codeword interleaving, followed by example details of how to compensatefor the bit shuffling. Further details of bit shuffling by the OTL canbe found in Appendix A below and in ITU G.709.

FIG. 6 shows how the OTL distributes 16-byte groups into a plurality oflogical lanes. Each 16-byte group is distributed, round robin, to eachlogical lane. In the illustrated example, it is assumed that there are20 logical lanes. However, other implementations are possible in whichthere may be a different number of logical lanes. In thisimplementation, each frame has 980 16-byte groups and there are 20logical lanes, so there are 49 16-byte groups distributed to eachlogical lane.

Logical lane assignments are rotated on each frame boundary. Thus,sequential frames differ in terms of how the 16-byte groups aredistributed. For each frame, the third OA2 byte position (i.e. sixthbyte position in the frame) is used to carry an LLM (Logical LaneMarker), which indicates how the 16-byte groups of the frame have beendistributed. The LLM increments on successive frames, from 0 to 239, butsince there are only 20 logical lanes, there are only 20 uniquedistributions. The logical lane number is recovered from the third OA2value by a “modulo 20” operation.

FIG. 7 shows bit-shuffling by the OTL from the logical lanes into 28Gphysical lanes. In the illustrated example, such bit-shuffling isaccomplished using four multiplexers (one for each physical lane). Eachmultiplexer performs bit-interleaving of data from five logical lanes(i.e. 5× codeword interleaving). While the interleaving is accomplishedusing four multiplexers, other implementations are possible in whichthere may be a different number of multiplexers.

The FEC parity calculation should compensate for the bit shuffling doneby the OTL so that we get striping at the symbol level across the four27G backplane lanes as shown in FIG. 7 and FIG. 8. Such striping canincrease tolerance against burst errors that may be experienced at theoutput of a DFE of the receiving device. Thus, as noted above withreference to FIG. 3, there is circuitry 312 for bit shuffling before theRS coding and circuitry 323 for bit re-shuffling after the RS coding inorder to compensate for the bit shuffling done by the OTL. In otherwords, the Reed Solomon parity symbols are computed over a bit-shuffledversion of the OTN backplane frame. In this specific embodiment, theparity calculation is with RS (627,613, t=7, m=10) over GF(2^10). Moregenerally, any suitable FEC parity calculation can be employed, whetherRS-based or not.

In specific implementations, the circuitry 312 for bit shufflingoperates over a 5×128 bits=640 bits=64 symbol window. Each group of5×128 bits is mapped into 64 10-bit symbols. The same bit shufflingpattern repeats every 5×128 bits=80 bytes=64 symbols. In specificimplementations, each group of 20×128 bits (bit[LL][n], LL=0 . . . 19,n=0 . . . 127) is mapped into 256 10-bit symbols (symbol[S][m], s=0 . .. 255, m=0 . . . 9) as follows:

-   -   sym[S][m]←bit[LL][n]        -   S=4*floor(n/2)+floor(LL/5)        -   m=5*(n mod 2)+(LL mod 5).            Furthermore, each group of 256 10-bit symbols (symbol[S][m],            s=0 . . . 255, m=0 . . . 9) is mapped into 20×128 bits            (bit[LL][n], LL=0 . . . 19, n=0 . . . 127) as follows:    -   bit[LL][n]←sym[S][m]        -   LL=5*(S mod 4)+(m mod 5)        -   n=2*floor(S/4)+floor(m/5).            Note that this is a very specific example and that other bit            shuffling schemes are within the scope of this disclosure.            More generally, any suitable bit shuffling that compensates            for the bit shuffling done by the OTL can be implemented.            Note that the OTL could interleave the logical lanes into            the electrical lanes in many different ways. Thus, there may            be corresponding bit shuffling/de-shuffling compensation            methods that can be implemented by the circuitry 312,314.            The example shown in FIG. 7 involves interleaving            LL0/LL1//L2/LL3/LL4 into electrical lane #0, but this could            be done in a different way. For example, in alternative            implementations, the OTL interleaves LL0/LL4/LL8/LL12/LL16            into electrical lane #0. This would involve a different bit            shuffling/de-shuffling compensation method by the circuitry            312,314.

In the alternative case of no codeword interleaving by the OTL, the FECparity calculation should still compensate for the bit shuffling done bythe OTL so that we get striping at the symbol level across the four 27Gbackplane lanes as shown in FIG. 9. Details of the bit shuffling done bythe OTL for the case of no codeword interleaving are not shown.

Note that Appendix II of G.709 fails to teach of a functionallystandardized OTUk with a smaller OTUkV FEC according to the presentdisclosure. Furthermore, Appendix II of G.709 fails to teach the RSencoding with the foregoing bit shuffling function that compensates forthe bit shuffling done by the OTL.

FEC Encoder/Decoder: Second Embodiment

According to a second embodiment, data of an OTN frame (for example, anOTN frame with a 255/239 FEC redundancy) is re-structured into anotherOTN frame (the backplane frame) with a 245.5/239 FEC redundancy. Thebackplane frame is structured similarly to a standard OTN frame: 9820byte×4 rows=7856 symbol×4 row. Ten rows on the incoming OTN frames aremapped into each backplane frame.

Fifteen Reed Solomon codewords are mapped into each row of the backplaneframe; this is, fourteen RS (528,514, m=10, t=7) codewords plus ashortened RS (464, 450, m=10, t=7) codeword.

A FIFO is used for the re-structuring. On each backplane frame row, thefifteen codewords can be mapped using one of two different methods:

-   -   By mapping the fifteen codewords on a sequential        (non-interleaved) fashion.    -   By interleaving the codewords at the symbol level.

Note that the re-structuring of data can reduce latency (even though theFIFO might introduce some latency) because there is less data to receivebefore processing with FEC parity can be started. Note that the FECparity from the incoming OTN frame is discarded, as new FEC parity forthe fifteen codewords is calculated with RS (Reed Solomon) because ofthe restructuring of the data.

The OTN FAS bytes (6×8×10/4=120 bits per BP frame) are “borrowed” tocarry the BP frame FAS (40 bits) and parity (80 bits):

-   -   Available parity=10×6.5×128 bits+80 bits [extra parity]=8,400        bits    -   Parity to use=15×4×140 bits=8,400 bits    -   Protected data=15×4×5140 bits=308,400 bits    -   Data to protect=10×239×128 bits−80 bits [extra parity]=305,840        bits        Note that (308,400−305,840)/10=256 symbols, so every backplane        frame row, one of the fifteen codewords is shortened by 256/4=64        symbols.

There are special considerations for the FEC parity calculation with RSbecause the OTL involves bit shuffling as explained for the firstembodiment. The FEC parity calculation should compensate for the bitshuffling done by the OTL so that we get striping at the symbol levelacross the four 27G backplane lanes. Examples are shown for the firstembodiment in FIGS. 7 and 8 for the case of 5× codeword interleaving andin FIG. 9 for the case of no codeword interleaving. The secondembodiment similarly entails striping at the symbol level across thefour 27G backplane lanes, with or without codeword interleaving.

Such striping can increase tolerance against burst errors that may beexperienced at the output of a DFE of the receiving device. Thus, asnoted above with reference to FIG. 3, there is circuitry 312 for bitshuffling before the RS coding and circuitry 323 for bit re-shufflingafter the RS coding in order to compensate for the bit shuffling done bythe OTL. In other words, the Reed Solomon parity symbols are computedover a bit-shuffled version of the OTN backplane frame. In this specificembodiment, the FEC parity calculation is with RS (528,514, t=7, m=10)over GF(2^10). More generally, any suitable FEC parity calculation canbe employed, whether RS-based or not.

In specific implementations, circuitry 312 for bit shuffling operatesover a 5×128 bits=640 bits=64 symbol window. Each group of 5×128 bits ismapped into 64 10-bit symbols. The same bit shuffling pattern repeatsevery 5×128 bits=80 bytes=64 symbols. In specific implementations, eachgroup of 20×128 bits (bit[LL][n], LL=0 . . . 19, n=0 . . . 127) ismapped into 256 10-bit symbols (symbol[S][m], s=0 . . . 255, m=0 . . .9) as follows:

-   -   sym[S][m]←bit[LL][n]        -   S=4*floor(n/2)+floor(LL/5)        -   m=5*(n mod 2)+(LL mod 5).            Furthermore, each group of 256 10-bit symbols (symbol[S][m],            s=0 . . . 255, m=0 . . . 9) is mapped into 20×128 bits            (bit[LL][n], LL=0 . . . 19, n=0 . . . 127) as follows:    -   bit[LL][n]←sym[S][m]        -   LL=5*(S mod 4)+(m mod 5)        -   n=2*floor(S/4)+floor(m/5).

Note that this is a very specific example and that other bit shufflingschemes are within the scope of this disclosure. More generally, anysuitable bit shuffling that compensates for the bit shuffling done bythe OTL can be implemented. Note that the OTL could interleave thelogical lanes into the electrical lanes in many different ways. Thus,there may be corresponding bit shuffling/de-shuffling compensationmethods that can be implemented by the circuitry 312,314. The exampleshown in FIG. 7 involves interleaving LL0/LL1/L2/LL3/LL4 into electricallane #0, but this could be done in a different way. For example, inalternative implementations, the OTL interleaves LL0/LL4/LL8/LL12/LL16into electrical lane #0. This would involve a different bitshuffling/de-shuffling compensation method by the circuitry 312,314.

Note that Appendix II of G.709 fails to teach of a functionallystandardized OTUk with a smaller OTUkV FEC according to the presentdisclosure. Furthermore, Appendix II of G.709 fails to teach the RSencoding with the foregoing bit shuffling function that compensates forthe bit shuffling done by the OTL.

FEC Encoder/Decoder: Third Embodiment

According to a third embodiment, data of an OTN frame (for example, anOTN frame with a 255/239 FEC redundancy) is re-structured into anotherOTN frame (the backplane frame) with a 242.5/239 FEC redundancy. Thebackplane frame is structured similarly to a standard OTN frame withfour rows and 2425×128 bits per row. Ten incoming OTN frames are mappedinto each backplane frame.

Thirty-two RS (970,956, m=10, t=7) codewords are mapped into each row ofthe backplane frame. A FIFO is used for the re-structuring. On eachbackplane frame row, the thirty-two RS codewords can be mapped using oneof two different methods:

-   -   By mapping the thirty-two codewords on a sequential        (non-interleaved) fashion.    -   By interleaving the codewords at the symbol level (4× codeword        interleaving, 8× codeword interleaving, etc.).

Note that the re-structuring of data can reduce latency (even though theFIFO might introduce some latency) because there is less data to receivebefore processing with FEC parity can be started. Note that the FECparity from the incoming OTN frame is discarded, as new FEC parity forthe 32 codewords (per backplane frame row) is calculated with RS (ReedSolomon) because of the restructuring of the data.

In this third embodiment, the amount of OTN data and parity bits matchthe number of bits on the RS codewords:

-   -   Available parity (per backplane row)=10×3.5×128 bits=4,480 bits    -   Parity to use (per backplane row)=32×140 bits=4,480 bits    -   Protected data (per backplane row)=32×9560 bits=305,920 bits    -   Data to protect (per backplane row)=10×239×128 bits=305,920 bits

A number of OTN FAS bytes (for example, one OTN FAS byte per OTN frame)is “borrowed” to carry the backplane frame FAS bytes.

There are special considerations for the FEC parity calculation with RSbecause the OTL involves bit shuffling as explained for the firstembodiment. The FEC parity calculation should compensate for the bitshuffling done by the OTL so that we get striping at the symbol levelacross the four 27G backplane lanes. Examples are shown for the firstembodiment in FIGS. 7 and 8 for the case of 5× codeword interleaving andin FIG. 9 for the case of no codeword interleaving. The third embodimentsimilarly entails striping at the symbol level across the four 27Gbackplane lanes, with or without codeword interleaving.

Such striping can increase tolerance against burst errors that may beexperienced at the output of a DFE of the receiving device. Thus, asnoted above with reference to FIG. 3, there is circuitry 312 for bitshuffling before the RS coding and circuitry 323 for bit re-shufflingafter the RS coding in order to compensate for the bit shuffling done bythe OTL. In other words, the Reed Solomon parity symbols are computedover a bit-shuffled version of the OTN backplane frame. In this specificembodiment, the FEC parity calculation is with RS (970,956, t=7, m=10)over GF(2^10). More generally, any suitable FEC parity calculation canbe employed, whether RS-based or not.

In specific implementations, the circuitry 312 for bit shufflingoperates over a 5×128 bits=640 bits=64 symbol window. Each group of5×128 bits is mapped into 64 10-bit symbols. The same bit shufflingpattern repeats every 5×128 bits=80 bytes=64 symbols. In specificimplementations, each group of 20×128 bits (bit[LL][n], LL=0 . . . 19,n=0 . . . 127) is mapped into 256 10-bit symbols (symbol[S][m], s=0 . .. 255, m=0 . . . 9) as follows:

-   -   sym[S][m]←bit[LL][n]        -   S=4*floor(n/2)+floor(LL/5)        -   m=5*(n mod 2)+(LL mod 5).            Furthermore, each group of 256 10-bit symbols (symbol[S][m],            s=0 . . . 255, m=0 . . . 9) is mapped into 20×128 bits            (bit[LL][n], LL=0 . . . 19, n=0 . . . 127) as follows:    -   bit[LL][n]←sym[S][m]        -   LL=5*(S mod 4)+(m mod 5)        -   n=2*floor(S/4)+floor(m/5).

Note that this is a very specific example and that other bit shufflingschemes are within the scope of this disclosure. More generally, anysuitable bit shuffling that compensates for the bit shuffling done bythe OTL can be implemented. Note that the OTL could interleave thelogical lanes into the electrical lanes in many different ways. Thus,there may be corresponding bit shuffling/de-shuffling compensationmethods that can be implemented by the circuitry 312,314. The exampleshown in FIG. 7 involves interleaving LL0/LL1/L2/LL3/LL4 into electricallane #0, but this could be done in a different way. For example, inalternative implementations, the OTL interleaves LL0/LL4/LL8/LL12/LL16into electrical lane #0. This would involve a different bitshuffling/de-shuffling compensation method by the circuitry 312,314.

Note that Appendix II of G.709 fails to teach of a functionallystandardized OTUk with a smaller OTUkV FEC according to the presentdisclosure. Furthermore, Appendix II of G.709 fails to teach the RSencoding with the foregoing bit shuffling function that compensates forthe bit shuffling done by the OTL.

Although specific embodiments have been disclosed in detail, it is to beunderstood that other embodiments are possible and are within the scopeof this disclosure. In general, systems and methods are provided formodifying an existing OTN frame (or creating a new frame with data fromthe OTN frame) prior to transmission. The precise manner in which thisis accomplished is implementation specific.

Example Highlights of Embodiments

Note that the foregoing embodiments may allow the backplane lane ratemay be reduced. For instance, a client OTN frame, based on the standard255/239 OTN frame structure, is mapped into an OTN backplane frame basedon a (lower rate) 245/239 frame structure (i.e.Backplane_frame_rate=OTN_frame_rate×245/255). Thus, the OTN backplanerate is approximately 4% lower than the OTN signal rate. This may beimportant for 28G electricals.

Additional example highlights are listed below.

-   -   compatible with ITU-T G.709 OTL4.n method (and the future OTLC.n        method) to allow hardware reuse    -   Supports the de-aggregation of the 27G lanes into 5G logical        lanes (as per ITU-T G.709 OTL4.4/OTL4.10 specification)    -   Different interleaving options are supported for the mapping of        the RS codewords into the OTN frame rows:        -   no codeword interleaving, to reduce latency.        -   codeword interleaving, interleaved at the symbol level, to            increase the tolerance against burst errors.

Table 1 shows Net Coding Gain of the OTN backplane FEC compared to thestandard G.709 RS FEC (GFEC).

TABLE 1 Net coding gain (in absence of DFE burst errors) BERin Codingrate NCG (at BERout = 10⁻¹⁵) OTN backplane FEC 2.51% 5.59 dB 1.89E−5(first embodiment) OTN backplane FEC 2.72% 5.65 dB 2.19E−5 (secondembodiment) OTN backplane FEC 1.46% 5.44 dB 1.19E−5 (this disclosure)GFEC 7% 6.7% 6.19 dB 8.28E−5 (G.709)From this chart it can be seen that the OTN backplane FEC can offerbenefits over the standard G.709 RS (255,239, t=8, m=8) FEC because itcan provide a good coding gain (5.59/5.65/5.44 dB vs. 6.19 dB) withcomparatively low coding rate (2.51/2.72/1.46% vs. 6.7%). This can beshown by a technical term called “Gap to Capacity”.

TABLE 2 compares the “gap to capacity” of the first embodiment of theOTN backplane FEC and the standard G.709 FEC (GFEC) at BERout=1E-12 andBERout=1E-15.

TABLE 2 Gap to capacity Gap to capacity BERout = 10⁻¹² BERout = 10⁻¹⁵OTN backplane FEC 2.75 dB 3.2 dB (-first embodiment) GFEC 7% 3.3 dB 3.78dB (G.709)The backplane lane rate is calculated as follows:

-   -   first embodiment: OTN backplane lane rate=OTN client        rate/4×245/255    -   second embodiment: OTN backplane lane rate=OTN client        rate/4×245.5/255    -   third embodiment: OTN backplane lane rate=OTN client        rate/4×242.5/255

TABLE 3 shows the 25G-class lane rate of the OTN backplane, for thefirst embodiment, for different base frequency options (i.e.SONET_rate×255/227, SONET_rate×255/226, etc.). The 255/227 ratiocorresponds to an OTU4 client signal. The 255/226, 255/225.5 and 255/225ratios correspond to potential candidates for the future OTUCn (“beyond100G” OTN rates).

TABLE 3 OTN backplane FEC (first embodiment) Lane rate for different OTNclient rate options OTN client rate = = OTN backplane lane SONET rate ×255/R rate = = OTN [R = 225, 226, 227, etc.] client rate × 245/255 SONET× 255/227 = 4 × 27.95249 Gbps 26.85632 Gbps SONET × 255/226 = 4 ×28.07618 Gbps 26.97515 Gbps SONET × 255/225.5 = 4 × 28.13843 Gbps27.03496 Gbps SONET × 255/225 = 4 × 28.20096 Gbps 27.09504 Gbps

A property of the present disclosure is that it supports both 10G and25G class lanes, since it still uses 5G logical lanes. By comparison,the 100GE backplane (802.3bj) cannot do that: the 25G lanes aremonolithic, do not consist of five 5G virtual lanes. This “property” isa result of reusing the OTL4.4 definition.

Appendix A

In general, OTLk.n allows OTUk (e.g. k=4) over n parallel lanes. As perG.709, OTLk.n refers to each of the n parallel lanes. For example,OTL4.4 refers to one of the four ˜27G lanes used to carry an OTU4.However, experts commonly use the term OTL (or OTLk.n) to refer to theentire parallel interface, or to refer to the “method” used to inversethe OTUk over a parallel interface (the OTL method). The OTN backplaneframe is structured similarly to an OTN frame (for example, 245 groupsof 128 bits per row, i.e. 245 column groups, for the first embodiment)and is inversed multiplexed over the four 27G backplane lanes by usingthe OTL4.4 method (ITU G.709).

The inverse multiplexing of an OTUk frame (with an OTUkV FEC having asmaller redundancy than the G.709 FEC) into n OTLk.n lanes causes thebit shuffling. Basically the OTN frame <-> nx OTL electrical B/P lanes.As defined in ITU G.709, the OTL4.4 function stripes the frame into20×128 bit groups (i.e. 20× column groups, each corresponding to adifferent 5G logical lane), and bit-interleaves five 5G logical lanesinto each of the four 27G OTN backplane lanes. FIG. 6 (corresponding tothe first embodiment), shows how the OTN backplane frame is inversedmultiplexed over twenty 5G logical lanes. FIG. 7 shows details of bitshuffling by the transmitting OTL. This Figure shows how sequential bitsof the OTN frame are mapped into the Reed Solomon symbols, and how theReed Solomon symbols are striped across the four 27G backplane lanes.

Most OTN acronyms used in this disclosure can be found in Section 4 ofITU-T G.709 recommendation (interfaces for the optical transportnetwork):

-   -   OTUk (Completely standardized Optical Channel Transport Unit-k):        -   The OTUk frame is the OTN transport frame with a            standardized OTN FEC (as the G.709 RS (255,239) FEC) that is            referred in this document    -   OTUkV (Functionally standardized Optical Channel Transport        Unit-k):        -   The OTUkV frame is the OTN transport frame with a non            standardized OTN FEC (as the other FEC schemes mentioned in            this document) that is referred in this document    -   OTL (Optical Channel Transport Lane):        -   An OTUk (or OTUkV) signal can be carried over a Multilane            interface with n OTLk.n (or OTLkV.n) electrical or optical            transport lanes/signals. The OTL terms mentioned in this            document can refer to either an OTLk(V).n lane or the whole            OTLk(V).n Multilane interface.            The GFEC acronym used in this disclosure refer to the            standard OTN G.709 RS (255,239) FEC (“G” is for G.709).

The OTUk (or OTUkV) transport frame can be inversely multiplexed (by 16Byte chunk granularity) into n OTLk.n (or OTLkV.n) Logical Lanes asspecified in Annex C of G.709 for k=4 and k=3. A similar scheme can beapplied to other values of k, such as k=Cn. Then, multiple OTLk.n (orOTLkV.n) Logical Lanes are bit-multiplexed into an OTLk.n (or OTLkV.n)Optical Transport Lane as shown in FIG. 6.5 and FIG. 8.3 of ITU-T G.709.FIGS. 6.5 and 8.3 of ITU-T G.709 along with their related descriptionsare expressly incorporated by reference.

For example, in the case of k=4 (OTU4, 111 809 973.568 kbit/s signal bitrate roughly), the value L=20 and the value n=4 (OTL4.4, 27 952 493.392kbit/Lane bit rate roughly, see table 7-5 or FIG. 6.5 and table 7-5 orITU-T G.709) or n=10 (OTL4.10, 11 180 997.3568 kbit/s lane bit rate seeAppendix VII of ITU-T G.709) are typically used (and specified) forOptical or Electrical lane transmission. In that essence, the OTU4signal is inversely multiplexed and carried through either four opticalor electrical lanes (OTL4.4) or ten optical or electrical lanes(OTL4.10). Table 7-5 of ITU-T G.709 along with its related descriptionare expressly incorporated by reference.

A goal of some embodiments is to carry an OTN frame signal with a lowerredundancy FEC than the G.709 RS (255,239) FEC over a Multilaneinterface with for example n electrical backplane lanes (OTLk(V).n). Forexample, in the case of k=4, a standard OTL4.4 Lane bit rate isapproximately 27 952 493.392 kbit/s (due to the 6.7% RS (255,239) FECredundancy) and by using some of the schemes and embodiments describedin this disclosure, an OTL4V.4 per lane transmission bit rate ofapproximately 26 856 317.805 kbit/s can be instead achieved (by usinginstead the roughly 2.5% RS FEC approach of the present disclosure, thatwould be an OTUkV FEC with a lower redundancy).

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the presentdisclosure may be practised otherwise than as specifically describedherein.

We claim:
 1. An OTN (Optical Transport Network) system comprising: anOTN transmitter comprising (i) an OTN framer device configured togenerate an OTN frame having FEC (Forward Error Correction) parity, (ii)an FEC encoder circuit configured to remove the FEC parity from the OTNframe, re-structure data from the OTN frame, create new FEC paritylocations, and generate FEC parity for the new parity locations therebygenerating an FEC encoded frame, and (iii) a transmitting OTL (Opticalchannel Transport Lane) configured to transmit the FEC encoded frameover a communication channel; an OTN backplane through which thecommunication channel traverses; and an OTN receiver comprising (i) areceiving OTL configured to receive the FEC encoded frame over thecommunication channel, and (ii) an FEC decoding circuit configured todecode the FEC encoded frame to recover the data; wherein the FECencoder circuit is configured to generate the FEC parity for the newparity locations in a manner that compensates for bit shufflingperformed by the transmitting OTL.
 2. The OTN system of claim 1, whereinthe FEC encoder circuit is configured to generate the FEC parity whilecompensating for bit shuffling done by the transmitting OTL such thatdata striping is achieved at a symbol level across physical lanes of thecommunication channel despite inverse multiplexing of the FEC encodedframe by the transmitting OTL.
 3. The OTN system of claim 2, wherein theFEC encoder circuit is configured to compensate for bit shuffling doneby the transmitting OTL by bit shuffling the data before generating theFEC parity and by bit de-shuffling after generating the FEC parity. 4.The OTN system of claim 3, wherein: bit shuffling the data beforegenerating the FEC parity comprises, for each group of 20×128 bits(bit[LL][n], LL=0..19, n=0..127), mapping the group into 256 10-bitsymbols (symbol[S][m], s=0..255, m=0..9) as follows: symbol[S][m]←bit[LL][n] S=4*floor(n/2)+floor(LL/5) m=5*(n mod 2)+(LL mod 5); and bitde-shuffling the data after generating the FEC parity comprises, foreach group of 256 10-bit symbols (symbol[S][m], s=0..255, m=0..9),mapping the group into 20×128 bits (bit[LL][n], LL=0..19, n=0..127) asfollows: bit[LL][n]←symbol[S][m] LL=5*(S mod 4)+(m mod 5)n=2*floor(S/4)+floor(m/5).
 5. The OTN system of claim 1, wherein the FECencoder circuit comprises an RS (Reed Solomon) encoder for generatingthe FEC parity.
 6. The OTN system of claim 5, wherein the FEC encodercircuit is configured to re-structure the OTN frame, which is initiallystructured with sixteen RS (255,239,m=8,t=8) per OTN frame row, into anew FEC structure having five RS (627,613,m=10,t=7) per OTN frame row.7. The OTN system of claim 5, wherein the FEC encoder circuit isconfigured to re-structure the OTN frame, which is initially structuredwith sixteen RS (255,239,m=8,t=8) per OTN frame row, into a new FECstructure having fifteen RS (528,514,m=10,t=7) per OTN frame row.
 8. TheOTN system of claim 5, wherein the FEC encoder circuit is configured tore-structure the OTN frame, which is initially structured with sixteenRS (255,239,m=8,t=8) per OTN frame row, into a new FEC structure havingthirty-two RS (970,956,m=10,t=7) per OTN frame row.
 9. The OTN system ofclaim 1, wherein the FEC encoder circuit is configured to remove FAS(Frame Alignment Signal) bytes from the OTN frame and insert new FASbytes into the OTN frame.
 10. A method for communicating data in an OTN(Optical Transport Network) system, the method comprising: generating anOTN frame having FEC (Forward Error Correction) parity; removing the FECparity from the OTN frame; re-structuring data from the OTN frame;creating new FEC parity locations; generating FEC parity for the newparity locations thereby generating an FEC encoded frame; transmitting,by a transmitting OTL (Optical channel Transport Lane), the FEC encodedframe over a communication channel; receiving the FEC encoded frame overthe communication channel; and decoding the FEC encoded frame to recoverthe data; wherein generating the FEC parity for the new parity locationsis performed in a manner that compensates for bit shuffling performed bythe transmitting OTL.
 11. The method of claim 10, comprising: generatingthe FEC parity while compensating for bit shuffling done by thetransmitting OTL such that data striping is achieved at a symbol levelacross physical lanes of the communication channel despite inversemultiplexing of the FEC encoded frame by the transmitting OTL.
 12. Themethod of claim 11, wherein compensating for bit shuffling done by thetransmitting OTL comprises: bit shuffling the data before generating theFEC parity; and bit de-shuffling after generating the FEC parity. 13.The method of claim 12, wherein: bit shuffling the data beforegenerating the FEC parity comprises, for each group of 20×128 bits(bit[LL][n], LL=0..19, n=0..127), mapping the group into 256 10-bitsymbols (symbol[S][m], s=0..255, m=0..9) as follows:symbol[S][m]←bit[LL][n] S=4*floor(n/2)+floor(LL/5) m=5*(n mod 2)+(LL mod5); and bit de-shuffling the data after generating the FEC paritycomprises, for each group of 256 10-bit symbols (symbol[S][m], s=0..255,m=0..9), mapping the group into 20×128 bits (bit[LL][n], LL=0..19,n=0..127) as follows: bit[LL][n]←symbol[S][m] LL=5*(S mod 4)+(m mod 5)n=2*floor(S/4)+floor(m/5).
 14. The method of claim 10, whereingenerating the FEC parity comprises an RS (Reed Solomon) encodergenerating the FEC parity.
 15. The method of claim 14, comprising:re-structuring the OTN frame, which is initially structured with sixteenRS (255,239,m=8,t=8) per OTN frame row, into a new FEC structure havingfive RS (627,613,m=10,t=7) per OTN frame row.
 16. The method of claim14, comprising: re-structuring the OTN frame, which is initiallystructured with sixteen RS(255,239,m=8,t=8) per OTN frame row, into anew FEC structure having fifteen RS (528,514,m=10,t=7) per OTN framerow.
 17. The method of claim 14, comprising: re-structuring the OTNframe, which is initially structured with sixteen RS(255,239,m=8,t=8)per OTN frame row, into a new FEC structure having thirty-two RS(970,956,m=10,t=7) per OTN frame row.
 18. The method of claim 10,further comprising: removing FAS (Frame Alignment Signal) bytes from theOTN frame; and inserting new FAS bytes into the OTN frame transmissionby the transmitting OTL.
 19. An OTN (Optical Transport Network)transmitter comprising: a framer device configured to generate an OTNframe having FEC (Forward Error Correction) parity; re-structuringcircuitry configured to remove the FEC parity from the OTN frame,re-structure data from the OTN frame, and create new FEC paritylocations; coding circuitry configured to generate FEC parity for thenew parity locations thereby generating an FEC encoded frame; and atransmitting OTL (Optical channel Transport Lane) configured to transmitthe FEC encoded frame; wherein the coding circuitry is configured togenerate the FEC parity in a manner that compensates for bit shufflingperformed by the transmitting OTL.
 20. The OTN transmitter of claim 19,comprising: an FPGA (Field Programmable Gate Array) implementing atleast the framer device.